New York City, hard hit by the collapse of some of Wall Streets most venerable firms, has a mayoral election scheduled for Nov. 3. With the citys fiscal woes certain to deepen in the next few years, there would seem to be a great deal to debate in advance of the vote. But neither the incumbent, Mayor Michael Bloomberg, nor his challenger, Comptroller William Thompson, has had much to say about the tsunami of expenditures—for pensions, programs, salaries and debt—that is poised to break over Gotham. In “Mike Bloomberg: Money, Power, Politics,” veteran New York Times journalist Joyce Purnick doesnt have much to say about it either.

And little wonder. Ms. Purnick, who had extensive access to the mayor and his staff, thinks that Mr. Bloomberg—a “benign plutocrat”—has been “one of the most effective mayors in the citys history.” Her book is mostly an admiring portrait of the man and his mayoralty.

Ms. Purnick met the future mayor, she tells us, in the late 1990s, when Mr. Bloomberg was hosting a dinner party at his Manhattan townhouse. She remembers seeing network news anchors Dan Rather and Peter Jennings at the gathering, but Mr. Bloomberg “barely made an impression.” Soon enough, though, she recognized that this self-made billionaire, who had amassed his fortune by providing information services to the financial industry, “is by nature a problem solver.”

Ms. Purnick sees Mr. Bloomberg as he would like to see himself. He turned to public life, she says, quoting him approvingly, because he “wanted to make a difference.” She describes him as relentlessly pragmatic, someone who exercises a “cold-eyed discipline over his frailties.” At his core, she claims, Mr. Bloomberg is not the ambitious office seeker who, for instance, hoped to buy his way onto the 2008 presidential ticket but rather a consummate manager who rises above the petty concerns of conventional politics. He decides matters, she says, “on the merits, yielding little to the customary political lobbyists, interest groups, and . . . campaign contributors, since there was only one—Bloomberg himself.”

When Ms. Purnick criticizes the mayor, it is generally because she perceives that he has failed his own better nature. Mr. Bloombergs recent coup de main—overthrowing the term-limits laws that he had once supported so that he can run for a third term—is for Ms. Purnick merely an aberration, a “detour to the dark side” that shows the mayor acting, this once, like a “selfish pol.” She gives only cursory coverage of the controversies of Mr. Bloombergs tenure, such as the record property-tax increase during his first term and the failed campaign to bring the Olympics to New York.

Its only near the end of “Mike Bloomberg” that Ms. Purnick briefly focuses on the issue that his mayoralty is likely to be judged by: his second-term fiscal stewardship. Mr. Bloomberg, she says, “followed the pattern established by other mayors.” Concerned with his re-election prospects, “he routinely granted all municipal union increases with no strings attached,” hoping to ensure “labor peace.” Astute reporter that she is, Ms. Purnick thus undercuts her own thesis about Mr. Bloombergs ability to behave as a consummate manager, not a garden-variety politician, though she doesnt seem to notice the contradiction.

It is certainly true that Mr. Bloomberg hasnt used his political power to aggrandize himself financially—there is no need for that. But he has used the public treasury for the greater Bloomberg glory. Starting with his first re-election run in 2005 and continuing through his current bid for a third term, Mr. Bloomberg has been in continuous campaign mode, adding to the city governments largess so much that New York had to increase its debt even when Wall Street was still pouring money into the citys coffers. And of course, thanks to the financial crisis, Wall Streets bounty can no longer be counted on. Spending has grown almost 50% on Mr. Bloombergs watch, while New Yorks annual pension obligation, based in part on the salary increases that Mr. Bloomberg negotiated, jumped to $6.3 billion from $1.4 billion a year. The mayors supposed pragmatism has produced the standard-issue outcome: New York is once again in fiscal peril.

Ms. Purnick seriously underplays this aspect of Mr. Bloombergs tenure and one other: In 2001, Mr. Bloomberg began his first campaign for mayor by pledging “to do for education what [Mayor Rudolph] Giuliani did for public safety.” His own mayoralty, he said, should be judged by what he accomplished in New Yorks schools. So what is the verdict? In “Mike Bloomberg,” Ms. Purnick keeps education off-stage until the last minute. She concludes that “improvement in the schools is indisputable.” But improvement in the students is another matter: Mr. Bloomberg spent an additional $8.5 billion on education, but the scores or city students on the SAT and NAEP (National Assessment of Educational Progress)—the two national tests not susceptible to local manipulation—remain flat.

If Michael Bloomberg is to achieve the greatness that Ms. Purnick is eager to confer on him already, he will have to do so in his third term, assuming that he is re-elected. The schools will always matter, of course, but the mayor might want to start by extricating the city from the fiscal hole he has dug for it. The “problem solver” created the problem; now he can solve it.